Since the advent of plasma spraying of metals and ceramics to form coatings on surfaces in the 1950s, the plasma spraying process has become very important commercially. Surprisingly, the apparatus used (the basic art geometry) has essentially remained the same.
Referring to FIG. 1, a conventional plasma spray torch 10' is illustrated. To simplify the disclosure, the water cooling means have purposely been eliminated from that figure. An electrically insulating body piece 10 of cylindrical, cup-shape form supports a cathode electrode 12 coaxially and projecting towards but spaced from a second body piece 11 closing off the interior of the electrically insulating body piece 10 at the end opposite that supporting the cathode electrode 12. The second body piece 11 is provided with an axial a bore 11a constituting the plasma spray torch nozzle passage 9. An arc 17 is formed by connecting an electrical potential difference across the cathode electrode 12 and the second body piece 11, acting as the anode. The arc 17 passes from the electrode 12 to the inner wall of the nozzle passage 9. Its length is extended by a flow of plasma-forming gas as shown by the arrow G which enters the annular manifold 24 about the cathode electrode 12 through a gas supply tube 15. Tube 15 connects to the body piece 10 and through an aligned radial hole 15a within the side of that cylindrical body piece. A transverse partition 13 of insulating material, like that of body piece 10, supports the electrode 12. The partition 13 is provided with a number of small diameter passages 23 leading into the nozzle passage 9 with flow about the tapered tip end 12a of the electrode 12. Powder to be sprayed as indicated by the arrow P, passes into the arc-heated gases at a point beyond the anode foot 18 of arc 17. Powder is introduced through the tube 16 and flows into a passage 16' aligned therewith and opening to the bore 11a in such a manner as to assure centering of the powder flow as best possible along the hot gas jet 25 which exits from the end of the nozzle 9.
An extremely bright conical arc region 19 extends a short distance beyond the exit of nozzle 9 with this region constituting the further extension of the ionized gas species. Tremendous heat transfer rates occur within the conical region 19. As may be appreciated, there is added gaseous heating of particle P flow beyond the ionized zone 19 within the hot gas jet 25. Further the particles pick up speed in the high velocity (but subsonic) jet 25 to strike the surface of the workpiece 22 and to form the coating 21 thereon.
Exemplary, the conventional plasma spray torch 10' is provided with a flow of 100 SCFH of nitrogen gas G using a nozzle passage 9 bore diameter of 5/16th of an inch, and the torch is provided with an operating current of 750 amp and an arc voltage of 80 volts. The ionized zone or region 19 is observed to extend about 1/3 of an inch beyond the end 9a of the nozzle. The gross power level reached is 60 Kw. The combined cathode and anode losses are about 30 volts with a net heating capability (I.sup.2 R heating of the gas) of 37.5 Kw. Assuming an additional heat loss to the cooling water of 20%, the gas heating amounts to 30 Kw. The enthalpy increase of the plasma gas in such conventional system under the conventional operating parameters set forth above is about 14,500 Btu per pound.
The Applicant has undertaken a detailed study of the beneficial effects of an extended high temperature supersonic flame cutting apparatus and method of rid transfer plasma arc torches, which study and results are exemplified by Applicant's recently issued U.S. Pat. No. 4,620,648 of Dec. 2, 1986. In conjunction with consideration of beneficial effects of extending the arc in nontransferred plasma arc torches, Applicant considered the utilization of a vortex flow of the plasma gas through the torch nozzle passage as facilitating the creation of an extended arc. In such considerations, the Applicant had full knowledge that in the past, vortex flow in nontransferred plasma-arc torches has led to a unreliable operation. Using subsonic jet velocities, the arc column bends back to strike the end face of the angled piece (such as the second body piece 11) in the conventional plasma arc spray torch 10' of FIG. 1 at points radially well removed from the nozzle 9a exit. Rapid torch erosion results.
In spite of this knowledge, applicant sought an improved, high voltage, high current extended ionized arc column nontransferred plasma arc torch that could be employed to direct particles at supersonic jet velocity with a short dwell time against a substrate to be coated with adequate melting of the particles ensured and without torch erosion.